Arsenic is a global threat to health, and one of the most commonly encountered contaminants in Superfund sites in the United States. Arsenic-exposed populations have been the focus of epidemiological studies that have found a diverse set of human diseases associated with arsenic exposure, including several forms of cancer, peripheral neuropathy, and severe peripheral vascular disease. A natural focus of epidemiological research has been to identify risk factors that predict the fraction of the exposed population that will contract arsenic-associated disease. Validated risk factors include the duration-weighted exposure level of arsenic, gender, nutritional status, genetic variations, and the efficiency of arsenic methylation during its metabolism. Understanding the effect and biological relevance of these risk factors has advanced the field, yet the epidemiological data suggest that there are still significant sources of disease risk that we have not yet identified. This proposal is based on the hypothesis that a key source of disease risk is individual variability in susceptibility to arsenic cytotoxicity, a phenomenon that has been observed in, as one example, limited studies of blood cells from arsenic-exposed humans. In this project we propose to utilize lymphoblastoid cell lines (LBLs) from a total of 130 individuals to characterize the individual variability in susceptibility to arsenic cytotoxicity. Genome-wide gene expression levels will be measured by RNA microarray analysis in order to identify genes whose expression levels correlate with arsenic-resistance level within this in vitro population. Candidate arsenic resistance genes will be subject to experimental modulation of gene expression levels in order to validate their functional significance in conferring arsenic resistance. Finally, a set of functionally validated candidate genes that identify the level of arsenic susceptibility will be tested in primary blood cells sampled from individuals at high arsenic exposure compared to a corresponding group of individuals at low arsenic exposure. The long-term goal of this project is twofold: to provide mechanistic information about genes that can reduce arsenic cytotoxicity and to develop additional biomarkers of arsenic-associated disease risk, allowed more refined assessment of risk to real-world populations.